Semiconductor device and method for producing the same

ABSTRACT

In the production of thin film transistor (TFT), a gate insulating film is formed to cover an active layer, a titanium nitride film is formed on the gate insulating film, and an aluminum film used as the gate electrode is formed on the titanium nitride film. The resulted configuration prevents the etching of the aluminum film from the insulating film side even if the etchant of aluminum enters the recessed portion at the edge of the active layer during the patterning of the gate electrode. Also in the anodizing process, when an oxide film is formed on the surface of the aluminum film, the oxidation of aluminum from the gate insulating film side is prevented even when the electrolyte solution enters the recessed portion at the edge of the active layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an insulated gate field effect semiconductordevice using a thin film semiconductor and a method for producing thesame.

2. Description of the Related Art

A known structure of an insulated gate field effect transistor using athin film semiconductor (hereinafter referred to simply as “TFT”) isshown in FIG. 2D. The method for producing such TFT is described belowwith reference to FIGS. 2A to 2D.

A base film (silicon oxide film) 22 is formed on a glass substrate 21 ata thickness of approximately 2000 Å. On the base film 22, a siliconsemiconductor layer 23 having amorphous or crystalline structure isformed as an active layer (where source/drain regions and a channelforming region are formed) at a thickness of about 1000 Å to obtain theshape shown in FIG. 2A. An element separation patterning is performed toobtain a shape shown in FIG. 2B. During this patterning, it is difficultto etch only the active layer 23, and the base film 22 is also etched tosome extent. As a result, a recessed portion 24 occurs on the base film22.

A silicon oxide film 26 as the gate insulating film is formed at athickness of approximately 1000 Å. As seen in FIG. 2C, however, the film26 also generates a recessed portion 27. FIG. 4 is a cross section TEMphotograph corresponding to the shape shown in FIG. 2C. The photographrepresents the state of thin film at the recessed portion 27 where aconcave strip appears to form a notch.

After this etching, an aluminum film 28 is formed at a thickness of 6000Å, and the film 28 is patterned to form a gate electrode. Then ananodizing treatment is given to the patterned electrode to form an oxidelayer 29 at a thickness of 2000 Å. FIG. 2D shows the A—A′ cross sectionof FIG. 2C. As illustrated in FIG. 2D, the aluminum film 28 is patternedto form the gate electrode. FIG. 3 is a schematic drawing of a plan viewof a TFT shown in FIG. 2C or FIG. 2D. The C—C′ cross section of FIG. 3corresponds to FIG. 2D, and the B—B′ cross section corresponds to FIG.2C. The reference numbers 30 through 32 in FIG. 3 are the contactelectrodes, though they are not shown in FIG. 2C and FIG. 2D.

A problem of such TFT is that the presence of recessed portion 27 causessubstantial break of the gate electrode and the gate wiring 28. Thebreaking is presumably caused by the following phenomena.

1. The patterning of the gate electrode 28 made of aluminum ispreferably conducted by a selective etching using a wet-etching method.By this etching process, however, an etchant solution enters into therecessed portion 27. As a result, the recess is enlarged, and, in theworst case, the gate electrode 28 breaks at the portion 34.

2. By anodizing after the aluminum film 28 is patterned, the surface ofthe patterned gate electrode 28 is oxidized. During the anodizing,however, the electrolyte solution enters into the recessed portion 27 tooxidize the portion 34 from the gate electrode side. Consequently, thegate electrode 28 increases its resistance and further becomesinsulated.

The defects of TFT are supposed often to occur by the combination ofthese reasons. The production of TFTs having a structure shown in FIGS.2C to 2D faces reduction of a yield.

SUMMARY OF THE INVENTION

The object of this invention is to prevent the etching of the portion 27during the patterning of the aluminum film 28 and to prevent theoxidation also at the recessed portion 27 during the anodization afterthis patterning, in the treatment shown in FIG. 2C.

A preferred mode of the invention is described using FIG. 1C. Accordingto the invention, as typically illustrated in FIG. 1C, a titaniumnitride film 17 is formed on a gate insulating film 16, and further analuminum film 18 used as a gate electrode is formed on the film 16.Since the titanium nitride film 17 can be formed at an extremely highstep coverage (difference level coating) using a sputtering method, therecess 151 is buried or covered by the film 17.

The reason for selecting the titanium nitride film is that the materialhas etching selectivity to aluminum film. In concrete terms, during theetching of aluminum film, the titanium nitride film is not etched, andduring anodizing, the titanium nitride film is not oxidized.Accordingly, a film having those characteristics may be used in place oftitanium nitride film independent of the conductivity and insulatingproperty of the film. Examples of that type of material that exhibits asimilar effect as the titanium nitride film and is useful in thisinvention are a metallic titanium film in which no nitrogen is added anda phosphorus-doped silicon film formed by low pressure CVD (LPCVD)method. That type of film may be formed at a thickness from 50 to 1000Å, for example, a thickness from 50 to 500 Å. It is necessary to formthe thin film in consideration of the thickness of gate insulating filmand of gate electrode.

According to the invention, wiring containing mainly aluminum is formedwith high reliability on an object having a convex portion. When anelectrode or wiring containing mainly aluminum is formed by covering onor crossing over the object (for example, the active layer 14 in FIG.1B) having a convex, the breakage of the wiring structured with thealuminum film 18 at the edge 15 of the convex portion can be prevented.Because the first film (for example, the titanium nitride film 17)exhibits an etching selectivity for the second film (for example, thealuminum film 18). This utilizes the characteristic that the first filmis not etched or has a low etching rate during the etching of secondfilm.

In particular, when a titanium nitride film is used as the first film,when an aluminum film is used as the second film, and when the surfaceof the aluminum film is oxidized during the anodization, the oxidationin the vicinity (the portion 152 of FIG. 1C) of edge root of the convexon the object from the object side can be prevented, and the defectscaused by substantial breakage of the second film can be reduced.

Titanium nitride film is not etched by an aluminum etchant.Consequently, even when an etchant enters the recess 151 during thepatterning of gate electrode and gate wiring, the etching of aluminumfilm 18 can be prevented.

Furthermore, in the anodizing after the patterning of aluminum film 18,even if the electrolyte solution enters the portion 151, the oxidizationof the aluminum film 18 from the gate insulating film side can beprevented.

As described above, the breakage of the aluminum film can be preventedby forming on the gate insulating film an aluminum film used as the gateelectrode, via a titanium nitride film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D show the procedure for producing a TFT of an embodimentof this invention.

FIGS. 2A to 2D show the procedure for producing a common TFT.

FIG. 3 shows a plan view of the structure of TFT in FIGS. 2C and 2D.

FIG. 4 is a photograph showing a cross section of the thin film in FIG.2C.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A to 1D illustrate the procedure for producing an N-channel thinfilm transistor (TFT) of an embodiment of this invention. Although theembodiment describes the production procedure for an N-channel TFT, aP-channel TFT may be applied for this invention. The TFT of theembodiment is applicable for TFTs formed at a peripheral circuit or at apixel area in a liquid crystal display unit, and further for TFTs inintegrated circuits.

The silicon oxide film 12 as the base film is formed at a thickness of2000 Å on the glass substrate 11 using a sputtering method. Theamorphous silicon film 13 is formed at a thickness of 1000 Å by plasmaCVD method. The formed silicon film 13 is crystallized by heating,irradiating laser beam, irradiating intense light, etc. A crystallinesilicon film may be formed directly by vapor deposition method insteadof the amorphous silicon film 13, and the amorphous film may be leftnon-crystallized. The element separation patterning is conducted to formthe active layer 14. The active layer 14 has the source/drain regionsand the channel forming region of TFT. During the element separationpatterning, a mask is placed on the crystallized silicon film 13 toperform anisotropic dry etching by the reactive ion etching (RIE)method. However, it is technically difficult to stop the dry etching atthe surface of the base film 12, and the etching actually proceeds intothe base film 12. In particular, the edge of the active layer 14 isnotched, and the etching also proceeds in the horizontal direction. As aresult, the portion 15 produces.

The silicon oxide film 16 as the gate insulating film is formed at athickness of 1000 Å by a sputtering method. As seen in FIG. 1C, a recessdirected towards the active layer 14 is formed in the silicon oxide film16.

It is useful to improve the characteristics of the interface between thesilicon oxide film 16 and the silicon film 14 used as the active layerby radiation of intense light after the silicon oxide film 16 is formed.As an example of the intense light, infrared light having a peak wavelength of 1.3 μm can be used. Radiation of such infrared light reducesthe interface level between the silicon film 14 and the silicon oxidefilm 16.

The next process is to form the titanium nitride film 17 at a thicknessof, for example, 500 Å in a range of from 50 to 1000 Å by a sputteringmethod. The sputtering method may be sputtering of a titanium nitridetarget by argon molecules or sputtering of a titanium by nitrogenmolecules. The conditions of the sputtering are: a pressure of 3×10-3Torr, RP power of 200 W, heating temperature of 200° C.

The titanium nitride film 17 is formed to bury or cover the recessedportion 151 on the silicon oxide film 16. Then, the aluminum film 18 isformed. The aluminum film 18 is patterned by a wet etching method. Amixed acid aluminum solution is used in the patterning to selectivelyetch only the aluminum film 18. Since the titanium nitride film 17 isnot almost etched, the recess portion 151 does not enlarge. For thesmooth operation of the succeeding anodization, one or more of theelements selected from scandium, palladium, and silicon is added to thealuminum film 18 at 1 to 5% by weight.

The titanium nitride film 17 is etched with an aqueous ammonia(NH₃OH/H₂O₂/H₂O) using the patterned aluminum film 18 as a mask. Duringthe treatment, the titanium nitride film 17 is selectively etched. Then,the oxide layer 19 is formed on the surface of the aluminum film 18, asthe gate electrode patterned by anodization. In the anodization, theelectrolyte solution of ethylene glycol containing tartaric acid (3% byweight) and ammonia water (5% by weight) is used. The thickness of theoxide layer formed by the treatment can be controlled by adjusting theapplied voltage and the oxidation time.

During the anodization, even if the electrolyte solution enters therecessed portion 151, the portion 152 is not oxidized from the side ofthe gate insulating film 16. Accordingly, the portion 152 becomes aninsulator and breaking of the aluminum film 18 as the gate electrode canbe prevented.

FIG. 1D shows the A—A′ cross section of FIG. 1C. The gate electrode 18and the oxide layer 19 are used as the mask to perform the phosphorus(P) ion implantation. In the ion implantation, phosphorus ions areimplanted into the regions 101 and 103, and the source/drain regions 101and 103 and the channel forming region 102 are formed in a self-aligningmanner. In this process, the thickness of the oxide layer 19 formedduring the anodization allows to form an offset gate region. Theactivation of the source/drain regions 101 and 103 is performed byradiation of laser light or intense light.

During the activation treatment (annealing), it is useful to apply rapidthermal annealing (RTA) using infrared light as the intense light. Theannealing method utilizes the selective absorption of infrared light tothe silicon film, to heat the silicon film up to 1000 to 1200° C. withina short time.

The film formation of interlayer insulators, the patterning for openingholes, and the formation of source/drain electrode and gate electrodeare performed to complete the production of TFT, though these processesare not shown in the figures.

When the titanium nitride film is formed on the gate insulating film,and when the aluminum film is formed on the titanium nitride film, thesucceeding patterning of the aluminum film does not induce the etchingof the aluminum film at the edge of the active layer from the gateinsulating film side. Furthermore, during the formation of the oxidelayer in the anodization, the progress of the oxidation of the aluminumfilm from the gate insulating film side is suppressed. Consequently, thesubstantial breakage of the gate wiring is prevented, and a TFT with ahigh reliability is produced.

What is claimed is:
 1. An insulated gate field effect transistorcomprising: an insulating film comprising silicon oxide; a semiconductorisland formed on said insulating film, said semiconductor islandincluding source and drain regions therein and a channel regiontherebetween; a gate insulating film formed over the semiconductorisland, said gate insulating film including silicon oxide; a metalnitride film formed on and in direct contact with the gate insulatingfilm; a metal film formed on and in direct contact with the metalnitride film, wherein the metal nitride film extends beyond edges of thesemiconductor island.
 2. A transistor according to claim 1, wherein themetal nitride is a titanium nitride film.
 3. A transistor according toclaim 1, wherein the metal nitride film has a thickness in a range of 50to 1000 Å.
 4. A transistor according to claim 1, wherein the metal filmis an aluminum film.
 5. A transistor according to claim 1 wherein asurface of said metal film is oxidized.
 6. A thin film transistorcomprising: an insulating film comprising silicon oxide; a semiconductorisland formed on said insulating film, said semiconductor islandincluding source and drain regions therein and a channel regiontherebetween; a gate insulating film formed over the semiconductorisland, said gate insulating film including silicon oxide; a gateelectrode formed on the gate insulating film, said gate electrodeincluding a metal nitride film formed on the gate insulating film and ametal film formed on the metal nitride film, wherein the metal nitridefilm extends beyond edges of the semiconductor island.
 7. A transistoraccording to claim 6, wherein the metal nitride is a titanium nitridefilm.
 8. A transistor according to claim 6, wherein the metal nitridefilm has a thickness in a range of 50 to 1000 Å.
 9. A transistoraccording to claim 6, wherein the metal film is an aluminum film.
 10. Atransistor according to claim 6 wherein a surface of said metal film isoxidized.
 11. A thin film transistor comprising: a semiconductor islandformed on an insulating surface, said semiconductor island includingsource and drain regions therein and a channel region therebetween; agate insulating film formed over the semiconductor island; a gateelectrode formed over the channel region with the gate insulating filminterposed therebetween, said gate electrode including a metal nitridefilm formed on the gate insulating film and a metal film formed on themetal nitride film, wherein the gate electrode extends beyond edges ofthe semiconductor island and said metal nitride film extends beyond sideedges of the metal film in a direction along said source and drainregions.
 12. A transistor according to claim 11, wherein the metalnitride is a titanium nitride film.
 13. A transistor according to claim11, wherein the metal nitride film has a thickness in a range of 50 to1000 Å.
 14. A transistor according to claim 11, wherein the metal filmis an aluminum film.
 15. A transistor according to claim 11 wherein asurface of said metal film is oxidized.
 16. A thin film transistorcomprising: an insulating film comprising silicon oxide; a semiconductorisland formed on said insulating film, said semiconductor islandincluding source and drain regions therein and a channel regiontherebetween; a gate insulating film formed over the semiconductorisland; a gate electrode formed over the channel region with the gateinsulating film interposed therebetween, said gate electrode including ametal nitride film formed on the gate insulating film and a metal filmformed on the metal nitride film, wherein the gate electrode extendsbeyond edges of the semiconductor island and said metal nitride filmextends beyond side edges of the metal film in a direction along saidsource and drain regions.
 17. A transistor according to claim 16,wherein the metal nitride is a titanium nitride film.
 18. A transistoraccording to claim 16, wherein the metal nitride film has a thickness ina range of 50 to 1000 Å.
 19. A transistor according to claim 16, whereinthe metal film is an aluminum film.
 20. A transistor according to claim16 wherein a surface of said metal film is oxidized.
 21. A thin filmtransistor comprising: a semiconductor island formed on an insulatingsurface, said semiconductor island including source and drain regionstherein and a channel region therebetween; a gate insulating film formedover the semiconductor island; a gate electrode formed over the channelregion with the gate insulating film interposed therebetween, said gateelectrode including a titanium nitride film formed on the gateinsulating film and an aluminum film formed on the titanium nitridefilm, wherein the gate electrode extends beyond edges of thesemiconductor island and said titanium nitride film extends beyond sideedges of the aluminum film in a direction along said source and drainregions.
 22. A transistor according to claim 21 wherein a surface ofsaid metal film is oxidized.
 23. A thin film transistor comprising: aninsulating film comprising silicon oxide; a semiconductor island formedon said insulating film, said semiconductor island including source anddrain regions therein and a channel region therebetween; a gateinsulating film formed over the semiconductor island; a gate electrodeformed over the channel region with the gate insulating film interposedtherebetween, said gate electrode including a metal nitride film formedon the gate insulating film and a metal film formed on the metal nitridefilm, wherein the metal nitride film extends beyond edges of thesemiconductor island and said metal nitride film extends beyond sideedges of the metal film in a direction along said source and drainregions.
 24. A transistor according to claim 23, wherein the metalnitride is a titanium nitride film.
 25. A transistor according to claim23, wherein the metal nitride film has a thickness in a range of 50 to1000 Å.
 26. A transistor according to claim 23, wherein the metal filmis an aluminum film.
 27. A transistor according to claim 23 wherein asurface of said metal film is oxidized.